1. Field of the Invention
This invention relates to a heating apparatus of the induction heating type for heating an image formed on a recording material.
2. Related Background Art
A heating apparatus of the electromagnetic induction heating type causes a magnetic field to act on a stationary or movable electrically conductive member (an electromagnetic induction heat generating member, an induction magnetic member or a magnetic field absorbing electrically conductive member) and effects the heating of a material to be heated by heat generation by an eddy current generated in the electrically conductive member, and is effective, for example, as an image heating and fixing apparatus for heating a recording material (a material to be heated) bearing an unfixed toner image thereon in an image forming apparatus of the electrophotographic type, the electrostatic recording type, the magnetic recording type or like type to thereby fix the unfixed toner image as a permanently secured image.
Generally, in the image heating and fixing apparatus, a magnetic field is caused to act on the stationary or movable electrically conductive member and by the heat generation by an eddy current generated in the electrically conductive member, a recording material which is a material to be heated conveyed in direct or indirect contact with the electrically conductive member is heated and therefore, magnetic field generating means comprising a coil and a core is constructed in a direction intersecting with the conveying direction of the recording material.
It is usual to use as the core a ferrite core often used in a transformer or a choke coil.
The ferrite core which is a sintered article is difficult to mold with a small thickness and usually, a thin ferrite core has a thickness of the order of 3 to 5 mm. Since it has a certain degree of thickness, a complicated mechanism is usually not required as a mechanism for holding this core. However, when an electrical characteristic is taken into account, the ferrite core has the disadvantage in which it must be used with a minimum thickness which is conversely determined in molding (in spite of having a sufficient surplus in an effective cross-sectional area relative to necessary magnetic flux density). That is, the thickness of the core has been greater than the effective cross-sectional area considered from the required magnetic flux density, and has increased unnecessary space and also has increased the material used to thereby increase the cost.
Also, the ferrite core is a sintered article as previously described and is therefore weak to the shock of a collision, a fall or the like and may be cracked or chipped. If it is used while being cracked or chipped, the deterioration of its characteristic or heat generation or the like will be caused and it will become necessary to take care in handling it in the manufacturing process thereof.
The present invention has been made in view of the above-noted problem and an object thereof is to provide a compact image heating apparatus of the induction heating type.
Another object of the present invention is to provide an image heating apparatus of the induction heating type which is high in the strength of a core.
Still another object of the present invention is to provide an image heating apparatus of the induction heating type in which the heat generation of a coil can be suppressed.
Yet still another object of the present invention is to provide an image heating apparatus of the induction heating type comprising:
a heating member;
an excitation coil for generating a magnetic field to induce an eddy current in the heating member; and
a core disposed in a magnetic circuit generated by the coil, the core being of a construction in which a plurality of sheet-shaped magnetic members are piled up.
Further objects of the present invention will become apparent from the following detailed description when read with reference to the accompanying drawings.